Learn Spanish (Spain): Yo que tú aplicaría una capa de cera porque es lo mejor. - If I were you, I would apply a layer of wax because it is best.

This is a very common idiomatic phrase in Spain. Literally, it translates to something like 'I than you,' but it functions just like the English phrase 'If I were you.' It is always used to give advice and is followed by the conditional tense.

Doesn't 'if I were' usually require the past subjunctive? Why is aplicaría in the conditional tense?

You are right that the full phrase si yo fuera tú (if I were you) uses the past subjunctive. However, the shortcut phrase yo que tú bypasses the 'if' clause entirely. Because you are stating what you would do, you just use the conditional tense directly: aplicaría (I would apply).

Why does the sentence use lo mejor instead of el mejor or la mejor?

Here, lo is the neuter article. We use lo plus an adjective when we aren't referring to a specific masculine or feminine noun, but rather an abstract idea. Lo mejor simply means 'the best thing' or 'what is best' in general.

Could I use yo en tu lugar instead of yo que tú here?

Yes, absolutely! Yo en tu lugar (literally: I in your place) is entirely interchangeable with yo que tú. Both are standard ways to give advice and both are followed by the conditional tense.